How do I align a view to the plane intersecting selected vertices
just rotate the whole loop with R from the side view then grab, move, and scale the whole loop into place.  for the trunk to have a smoother curve, you're going to have to add more edge loops with Cntrl R.

Edit: part of the reason is your topology, i just edited your image a little to give it a more practical topology. you just gotta move your vertices around a little near the mouth and neck area.  take a look at the edited reference image below